Semiconductor laser device

ABSTRACT

A semiconductor laser device includes a laser diode provided on a semiconductor substrate, the laser diode including a first optical waveguide having a gain waveguide, a plurality of photodiodes, a first wavelength-selective filter having periodic transmission peaks, and a second wavelength-selective filter having periodic transmission peaks, the period of the transmission peaks of the second wavelength-selective filter being different from the period of the transmission peaks of the first wavelength-selective filter. Furthermore, two photodiodes among the plurality of photodiodes are optically coupled to the first optical waveguide through the first and second wavelength-selective filters, respectively.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor laser device.

2. Description of the Related Art

Japanese Unexamined Patent Application Publication Nos. 2006-245346 and 2008-066318 disclose wavelength-tunable semiconductor lasers used in optical communication systems. The semiconductor laser disclosed in Japanese Unexamined Patent Application Publication No. 2006-245346 includes a plurality of ring resonators having optical path lengths different from each other, wherein the plurality of ring resonators are optically connected to each other. The semiconductor laser also includes a light input/output unit configured to input and output a laser beam to and from the plurality of ring resonators. These ring resonators have free spectral ranges (FSRs) different from each other. Laser oscillation occurs at a wavelength at which the transmission peak wavelengths of the respective ring resonators overlap each other. Furthermore, the lasing wavelength can be changed by changing the transmission peak wavelengths of the ring resonators by injecting a current into the ring resonators. In this case, a so-called vernier effect is used for changing the lasing wavelength. Furthermore, the lasing wavelength is monitored and it is controlled on the basis of the monitoring results.

In the semiconductor laser disclosed in Japanese Unexamined Patent Application Publication No. 2008-066318, ring resonators are connected at the front and the back of a gain region. The FSRs of these ring resonators are different from each other, and laser oscillation occurs at a wavelength at which the transmission peak wavelengths of the ring resonators overlap each other. The transmission peak wavelengths of the ring resonators can be changed by injecting a current into the ring resonators. That is, as in the semiconductor laser disclosed in Japanese Unexamined Patent Application Publication No. 2006-245346, the lasing wavelength can be changed by injecting a current into the ring resonators using the vernier effect.

Kimoto et al., “Highly Reliable 40-mW 25-GHz×20-ch Thermally Tunable DFB Laser Module, Integrated with Wavelength Monitor”, Furukawa Review, No. 24, 2003, pp. 1-5 discloses a laser module provided with a wavelength-tunable DFB laser and a wavelength control element (wavelength locker). The wavelength locker in this laser module includes a prism, two photodiodes, and an etalon filter. In this laser module, the prism branches backlight of the DFB laser into two light beams. The etalon filter receives one of the light beams and selectively passes light of a certain wavelength. One of the photodiodes directly receives the light beams from the DFB laser through the prism. The other of the photodiodes receives the light passing through the etalon filter.

SUMMARY OF THE INVENTION

Recently, in the field of optical communication systems, the amount of information to be transmitted has been increasing. Consequently, wavelength-division multiplexing (WDM) optical transmission systems have been constructed. In such WDM optical transmission systems, a plurality of signal light beams having different wavelengths are transmitted in a single optical fiber. Furthermore, in order to further increase the amount of information to be transmitted, it is desired that the wavelength interval between signal light beams be further reduced. In the conventional WDM optical transmission systems, a plurality of semiconductor lasers having emission wavelengths different from each other are used as an optical transmitter. However, the emission wavelength of each of the semiconductor lasers varies depending on the temperature or a change with time. In order to suppress such a variation in the emission wavelength and to further reduce the wavelength interval between signal light beams, a wavelength-tunable semiconductor laser or a wavelength-locked laser semiconductor laser can be used. In this case, the emission wavelength of these semiconductor lasers is monitored and the emission wavelength is controlled to be constant on the basis of the monitoring results.

In the wavelength-tunable lasers disclosed in Japanese Unexamined Patent Application Publication Nos. 2006-245346 and 2008-066318, since the wavelength-tunable range is limited, the wavelength-tunable lasers merely deal with variations in the emission wavelength within a relatively narrow range. This is because any type of wavelength-selective filter such as a ring resonator used for the vernier effect has a narrow FSR, and the wavelength control is limited within this FSR. If the emission wavelength varies outside the FSR, the emission wavelength is shifted to a wavelength different from an emission wavelength that is originally set, and thus it is difficult to accurately control the emission wavelength.

A semiconductor laser device according to the present invention includes a laser diode provided on a semiconductor substrate, the laser diode including a first optical waveguide having a gain waveguide, a plurality of photodiodes, a first wavelength-selective filter having periodic transmission peaks, and a second wavelength-selective filter having periodic transmission peaks, the period of the transmission peaks of the second wavelength-selective filter being different from the period of the transmission peaks of the first wavelength-selective filter. Two photodiodes among the plurality of photodiodes are optically coupled to the first optical waveguide through the first and second wavelength-selective filters, respectively.

In the semiconductor laser device described above, light generated in the gain waveguide of the laser diode is output as a laser beam. A portion of this laser beam is detected by a plurality of photodiodes each optically coupled to the first optical waveguide. Two photodiodes among the plurality of photodiodes are optically coupled to the first optical waveguide through the first and second wavelength-selective filters, respectively. Therefore, a wavelength of the laser beam output from the laser diode may be detected by receiving the laser beam with these two photodiodes through the first and second wavelength-selective filters, respectively. In this semiconductor laser device, the wavelength of the laser beam output from the laser diode is controlled mainly on the basis of the intensity of light detected through the second wavelength-selective filter.

In the semiconductor laser device, at least one of the plurality of photodiodes is optically coupled directly to the first optical waveguide. Accordingly, light propagating in the first optical waveguide is directly detected by the photodiode. Therefore, an output power of light emitted from the laser diode can be monitored by using this photodiode which is directly coupled to the first optical waveguide. Furthermore, a transmittance of light passing through the first wavelength-selective filter or the second wavelength-selective filter is easily estimated on the basis of the light intensity detected by this photodiode which is directly coupled to the first optical waveguide.

In the semiconductor laser device, the period of the transmission peaks of the first wavelength-selective filter is different from the period of the transmission peaks of the second wavelength-selective filter. For example, the semiconductor laser device is configured so that the period of the transmission peaks of the first wavelength-selective filter is longer than the period of the transmission peaks of the second wavelength-selective filter. Accordingly, even when the emission wavelength is largely varied by an amount exceeding a wavelength corresponding to the repetition period of the second wavelength-selective filter, the variation can be recognized by monitoring the intensity of light passing through the first wavelength-selective filter to correct the variation in the wavelength. Thus, according to the semiconductor laser device described above, the original emission wavelength can be suitably maintained even in a state in which the emission wavelength significantly varies.

In the semiconductor laser device, the period of the transmission peaks of the first wavelength-selective filter is preferably more than two times the period of the transmission peaks of the second wavelength-selective filter.

In the semiconductor laser device, the first wavelength-selective filter may include a ring resonator. A ring resonator is an optical element that can realize various FSRs in accordance with the radius of the ring. A wide FSR can be easily obtained by decreasing the radius of the ring. Accordingly, the wavelength-transmittance characteristics of the first wavelength-selective filter in the present invention can be easily realized.

In the semiconductor laser device, the laser diode may further include a reflector region provided at an end of the first optical waveguide. The first wavelength-selective filter may be formed on the semiconductor substrate. Furthermore, the reflector region may be located between the first wavelength-selective filter and the first optical waveguide of the laser diode, and one end of the first wavelength-selective filter may be optically coupled to the first optical waveguide of the laser diode with the reflector region therebetween.

In this semiconductor laser device, light emitted from the gain waveguide of the laser diode propagates in the first optical waveguide and is reflected at the reflector region provided at an end of the first optical waveguide. The reflector region functions as one of optical reflectors forming a laser cavity of the laser diode. A laser beam is output from an end facet of the first optical waveguide, the end facet opposite the reflector region. A portion of light reaching the reflector region passes through the reflector region and reaches the first wavelength-selective filter. This light passes through the first wavelength-selective filter and is detected by a photodiode. That is, according to this semiconductor laser device, light can be suitably monitored from the end of the first optical waveguide, the end opposite the end facet from which the laser beam is output. In addition, since the first wavelength-selective filter is formed on the semiconductor substrate, this structure can also contribute to a reduction in size of the device. Furthermore, the reflector region is located between the first wavelength-selective filter and the first optical waveguide of the laser diode, and a cleaved facet is not used in this semiconductor laser device. Accordingly, the laser diode, the first wavelength-selective filter, and the photodiode can be easily integrated on a single substrate.

In the semiconductor laser device, the reflector region may include a pair of films having a refractive index lower than the refractive index of the first optical waveguide. The pair of films extends in an in-plane direction perpendicular to the optical axis of the first optical waveguide, and forms an optical reflector of the first optical waveguide. In this semiconductor laser device, the reflector region functioning as one of optical reflectors forming the laser cavity of the laser diode can be suitably formed between the first wavelength-selective filter and the first optical waveguide of the laser diode.

In the semiconductor laser device, the pair of films may be composed of at least one selected from silicon dioxide (SiO₂), silicon nitride (SiN), and a benzocyclobutene (BCB) resin. Since the refractive index of these materials is lower than the refractive index of semiconductors, a pair of films having a refractive index lower than the refractive index of the first optical waveguide can be suitably realized.

In the semiconductor laser device, the laser diode may further include a distributed Bragg reflector (DBR) region including a diffraction grating formed along the first optical waveguide and a plurality of electrodes arranged in the light-propagation direction of the first optical waveguide, the distributed Bragg reflector region being provided at one end side of the gain waveguide, a light transmission portion provided at another end side of the gain waveguide, the light transmission portion having a plurality of transmission peaks that periodically change with a predetermined period, and a phase control portion for controlling the optical path length of the first optical waveguide. The diffraction grating of the distributed Bragg reflector region has a period that changes in a light-propagation direction of the first optical waveguide, and the reflector region and the distributed Bragg reflector region form a laser cavity. With this structure, the laser diode can be formed as a wavelength-tunable laser. Consequently, the semiconductor laser device according to the present invention is suitable for the WDM optical transmission systems as a tunable light source.

In the semiconductor laser device, the second wavelength-selective filter and the photodiode that is optically coupled to the first optical waveguide through the second wavelength-selective filter may be provided separately from the semiconductor substrate. With this structure, various types of wavelength-selective filter can be used as the second wavelength-selective filter, and the period of the transmission peak wavelengths can be suitably determined. In this case, the second wavelength-selective filter is preferably an etalon filter. The FSR of an etalon filter is narrower than that of other wavelength-selective filters such as a ring resonator, and thus an etalon filter can be suitably used as the second wavelength-selective filter.

In the semiconductor laser device, the laser diode may have a distributed feedback (DFB) laser structure.

In the semiconductor laser device, the laser diode may further include an end facet disposed at one end of the first optical waveguide, and a DBR region including a diffraction grating formed along the first optical waveguide and a plurality of electrodes arranged in the light-propagation direction of the first optical waveguide, the distributed Bragg reflector region being provided at one end side of the gain waveguide. The diffraction grating of the distributed Bragg reflector region has a period that changes in a light-propagation direction of the first optical waveguide. In this case, the end facet disposed at the one end of the first optical waveguide and the distributed Bragg reflector region may form a laser cavity. Furthermore, the first wavelength-selective filter may be formed on the semiconductor substrate, and one end of the first wavelength-selective filter may be optically coupled to the first optical waveguide of the laser diode with the DBR region therebetween.

In the semiconductor laser device, the laser diode may further include an optical amplification region. The optical amplification region may be formed on the semiconductor substrate, and one end of the optical amplification region and the one end of the first wavelength-selective filter may be each optically coupled, by using an optical coupler, to the first optical waveguide of the laser diode with the DBR region therebetween. In this case, the first wavelength-selective filter is arranged in parallel with the optical amplification region through the DBR region. Accordingly, even when the output of a laser beam through the optical amplification region is interrupted, monitor light passing through the first wavelength-selective filter can be detected. Therefore, the emission wavelength can be suitably controlled even during the time when the output of a laser beam is interrupted by the optical amplification region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the structure of a semiconductor laser device according to a first embodiment of the present invention.

FIG. 2 is a plan view showing the structure of a semiconductor laser.

FIG. 3 is a cross-sectional side view of the semiconductor laser taken along line III-III in FIG. 2.

FIG. 4A is a cross-sectional view of a semiconductor laser region taken along line IVa-IVa in FIG. 3 and shows the structure of a gain region and an optical amplification region.

FIG. 4B is a cross-sectional view of the semiconductor laser region taken along line IVb-IVb in FIG. 3 and shows the structure of a distributed Bragg reflector (DBR) region.

FIG. 5A is a graph showing an example of an emission spectrum of the gain region.

FIG. 5B is a graph showing an example of a reflection spectrum of the DBR region obtained when a current is supplied from one of a plurality of anode electrodes to an optical waveguide layer and a diffraction grating layer.

FIG. 6A is a cross-sectional view of the semiconductor laser region taken along line VIa-VIa in FIG. 3 and shows the structure of a ring resonator region and a phase control region.

FIG. 6B is a graph showing an example of a transmission spectrum of the ring resonator region.

FIG. 7A is an enlarged cross-sectional side view of a reflector region shown in FIG. 3.

FIG. 7B is a cross-sectional view of the reflector region taken along line VIIb-VIIb in FIG. 7A.

FIG. 8 is a graph showing an example of a reflection spectrum of a film structure including a pair of films in the reflector region.

FIG. 9A is a plan view showing the structure of a wavelength-monitoring region.

FIG. 9B is a cross-sectional view of the wavelength-monitoring region taken along line IXb-IXb in FIG. 9A and shows the structure of one of a plurality of photodiode structures in the wavelength-monitoring region.

FIG. 9C is a view showing the structure of a ring resonator.

FIG. 10 is a graph showing an example of transmission spectra of a ring resonator and an etalon filter in the wavelength-monitoring region.

FIG. 11 is a block diagram that schematically shows a control system for controlling the wavelength of a laser beam output from the semiconductor laser.

FIG. 12 is a graph that schematically illustrates the processing of a control circuit.

FIG. 13 is a plan view showing the structure of a semiconductor laser included in a semiconductor laser device according to a second embodiment of the present invention.

FIG. 14 is a cross-sectional side view of the semiconductor laser taken along line XIV-XIV in FIG. 13.

FIG. 15 is a block diagram that schematically shows a control system for controlling the wavelength of a laser beam output from the semiconductor laser shown in FIG. 13.

FIG. 16 is a plan view showing the structure of a semiconductor laser 2C included in a semiconductor laser device according to a third embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Semiconductor laser devices according to embodiments of the present invention will be described in detail below with reference to the accompanying drawings. In descriptions of the drawings, the same elements are assigned the same reference numerals and an overlapping description is omitted.

First Embodiment

A semiconductor laser device according to a first embodiment of the present invention will now be described. Referring to FIG. 1, a semiconductor laser device 1 according to this embodiment includes a semiconductor laser 2A, a lens 3, an optical isolator 4, a beam splitter 5, an etalon filter 6, and a photodiode element 7.

The semiconductor laser 2A is formed by integrating a wavelength-tunable semiconductor laser region (laser diode) that generates a laser beam L_(a) of any wavelength, and a wavelength-monitoring region for monitoring the wavelength of the laser beam L_(a) on a single semiconductor substrate.

The lens 3 is an optical component for collimating the laser beam L_(a) emitted from the semiconductor laser 2A and is optically coupled to a light-emitting end of the semiconductor laser 2A. The optical isolator 4 is an optical component for removing reflected light of the laser beam L_(a) returned to the semiconductor laser 2A. An input end of the optical isolator 4 is optically coupled to the light-emitting end of the semiconductor laser 2A with the lens 3 therebetween. The beam splitter 5 is optically coupled to an output end of the optical isolator 4 and divides the laser beam L_(a) emitted from the semiconductor laser 2A into two laser beams L_(out) and L_(mon). The laser beam L_(out) is output to an optical fiber F coupled to the semiconductor laser device 1. The laser beam L_(mon) passes through the etalon filter 6 and is then incident on a light-receiving surface of the photodiode element 7.

The etalon filter 6 is a second wavelength-selective filter in this embodiment and has periodic transmission peak wavelengths. The etalon filter 6 is optically coupled between the photodiode element 7 and the light-emitting end of the semiconductor laser 2A. The photodiode element 7 constitutes a part of a plurality of photodiodes included in the semiconductor laser device 1. The photodiode element 7 receives the laser beam L_(mon) having a certain wavelength and passing through the etalon filter 6, and outputs an electrical signal corresponding to the optical intensity of the laser beam L_(mon). Note that this electrical signal is provided to a control circuit provided outside the semiconductor laser device 1 and is used to control the emission wavelength of the semiconductor laser region of the semiconductor laser 2A. In this embodiment, the etalon filter 6 and the photodiode element 7 are provided as separate optical elements independent from the semiconductor laser 2A and differ from the wavelength-monitoring region in this point, the wavelength-monitoring region being provided on the semiconductor substrate of the semiconductor laser 2A.

Next, the inner structure of the semiconductor laser 2A will be described. Referring to FIGS. 2 and 3, the semiconductor laser 2A includes a semiconductor laser region 10 and a wavelength-monitoring region 20. Furthermore, the semiconductor laser region 10 and the wavelength-monitoring region 20 are formed by being integrated on a single semiconductor substrate. The semiconductor laser region 10 has a wavelength-tunable semiconductor laser structure. The semiconductor laser region 10 includes a gain region 10 a, a distributed Bragg reflector (DBR) region 10 b, an optical amplification region 10 c, a ring resonator region 10 d, a phase control region (phase control portion) 10 e, and a reflector region 10 f. In the wavelength-monitoring region 20, light passing through the reflector region 10 f of the semiconductor laser region 10 is branched into, for example, two light beams propagating in optical waveguides 212 and 213. One of the branched light beams propagating in the optical waveguide 213 passes through a ring resonator and is connected to a photodiode structure 22 b. The other of the branched light beams propagating in the optical waveguide 212 is directly connected to another photodiode structure 22 a. The monitoring result obtained from the wavelength-monitoring region 20 is provided to a control circuit provided outside the semiconductor laser device 1 and is used to control the emission wavelength of the semiconductor laser region 10.

The semiconductor laser region 10 and the wavelength-monitoring region 20 are formed on a single semiconductor substrate 8 as shown in FIG. 2, and are arranged in a light-propagation direction. Preferable lengths of the semiconductor laser region 10 and the wavelength-monitoring region 20 in the light-propagation direction are 1,500 μm and 1,000 μm, respectively. The semiconductor substrate 8 is composed of, for example, n-type InP and functions as a lower cladding of an optical waveguide in the semiconductor laser region 10 and the wavelength-monitoring region 20.

In addition, the semiconductor laser 2A is mounted on a temperature control device 30. The temperature control device 30 includes, for example, a Peltier device (thermoelectric cooling device). The temperature of the semiconductor substrate 8 and a stacked structure provided on the main surface of the semiconductor substrate 8 can be controlled to a certain temperature using the temperature control device 30.

First, the structure of the gain region 10 a, the DBR region 10 b, and the optical amplification region 10 c in the semiconductor laser region 10 will be described with reference to FIGS. 2, 3, 4A, and 4B. FIG. 4A is a cross-sectional view of the semiconductor laser region 10 taken along line IVa-IVa in FIG. 3 and shows the structure of the gain region 10 a and the optical amplification region 10 c. FIG. 4B is a cross-sectional view of the semiconductor laser region 10 taken along line IVb-IVb in FIG. 3 and shows the structure of the DBR region 10 b. In FIGS. 4A and 4B, the illustration of the temperature control device 30 shown in FIG. 3 is omitted.

Each of the gain region 10 a and the optical amplification region 10 c includes an optical waveguide layer 110 provided on the semiconductor substrate 8, an upper cladding layer 112 provided on the optical waveguide layer 110, and a contact layer 113 provided on the upper cladding layer 112. The optical waveguide layer 110 constitutes a part of a first optical waveguide in this embodiment. In particular, the optical waveguide layer 110 of the gain region 10 a corresponds to a gain waveguide (i.e., waveguide having an optical gain) in this embodiment. The optical waveguide layer 110 is composed of a semiconductor having a band-gap wavelength longer than (i.e., having a band-gap energy smaller than) that of the semiconductor substrate 8 and extends in the light-propagation direction along the main surface of the semiconductor substrate 8. Here, the band-gap wavelength λg (μm) and the band-gap energy Eg (eV) are related by the relationship λg=1.24/Eg. The optical waveguide layer 110 includes, for example, a lower optical confinement layer, an upper optical confinement layer and an active layer provided between the lower optical confinement layer and the upper optical confinement layer.

In an example, each of the lower optical confinement layer and the upper optical confinement layer is composed of undoped GaInAsP, and the active layer has a GaInAsP multi-quantum well (MQW) structure. The composition of the active layer is adjusted so that light with a wavelength of, for example, 1.52 to 1.57 μm is emitted (or amplified). FIG. 5A is a graph showing an example of an emission spectrum when a current is injected into the gain region 10 a. In FIG. 5A, the vertical axis represents the optical intensity, and the horizontal axis represents the wavelength. As shown in FIG. 5A, light over a relatively wide wavelength range is emitted from the gain region 10 a. That is, the gain region 10 a has an optical gain over a relatively wide wavelength range corresponding to the emission spectrum. The optical waveguide layer 110 has a thickness of, for example, 0.3 μm.

The upper cladding layer 112 is composed of p-type InP, and the contact layer 113 is composed of p-type GaInAs. The upper cladding layer 112 and the contact layer 113 have thicknesses of, for example, 2 μm and 0.2 μm, respectively.

The optical waveguide layer 110, the upper cladding layer 112, and the contact layer 113 have a stripe-shaped mesa structure on the semiconductor substrate 8, the stripe-shaped mesa structure extending in the light-propagation direction (refer to FIG. 4A). The width of the stripe-shaped mesa structure in a direction intersecting with the light-propagation direction is, for example, 1.5 μm. A semi-insulating portion 102 is provided on each side face of this stripe-shaped mesa structure. The semi-insulating portion 102 is composed of a semi-insulating semiconductor, for example, Fe-doped InP. The semi-insulating portion 102 is also provided on a portion of the semiconductor substrate 8, the portion being other than the stripe-shaped mesa structure.

An anode electrode 114 is provided on the contact layer 113 of the gain region 10 a, and an anode electrode 115 is provided on the contact layer 113 of the optical amplification region 10 c. The anode electrodes 114 and 115 are ohmic electrodes composed of, for example, Ti/Pt/Au. A cathode electrode 9 is provided on the reverse face of the semiconductor substrate 8. The cathode electrode 9 is an ohmic electrode composed of, for example, AuGe. A current is injected into the optical waveguide layer 110 through the anode electrodes 114 and 115 and the cathode electrode 9. Portions of the upper surfaces of the gain region 10 a and the optical amplification region 10 c, the portions being other than those of the anode electrodes 114 and 115, are covered with an insulating film 11 composed of, for example, SiO₂.

As shown in FIGS. 2 and 3, an anti-reflection (AR) film 105 is provided on an end facet of the optical amplification region 10 c in the light-propagation direction (i.e., on the light-emitting end facet of the semiconductor laser region 10). The reflectivity of this AR film 105 is, for example, 0.1%.

The DBR region 10 b is provided at one end side of the optical waveguide layer 110 of the gain region 10 a. A reflection spectrum of the DBR region 10 b has a plurality of reflectivity peaks that periodically change with a predetermined period.

The DBR region 10 b includes an optical waveguide layer 120 provided on the semiconductor substrate 8, a diffraction grating layer 121 provided on the optical waveguide layer 120, an upper cladding layer 122 provided on the diffraction grating layer 121, and a contact layer 123 provided on the upper cladding layer 122. The optical waveguide layer 120 constitutes a part of the first optical waveguide in this embodiment. The band-gap wavelength of the optical waveguide layer 120 is shorter than the band-gap wavelength of the active layer of the gain region 10 a and is, for example, 1.3 μm or less. The refractive index of the optical waveguide layer 120 changes in accordance with the magnitude of the current injected into the optical waveguide layer 120 through a cathode electrode 9 and anode electrodes 124 described below. The optical waveguide layer 120 is composed of a semiconductor having a band-gap wavelength longer than that of the semiconductor substrate 8. The optical waveguide layer 120 extends in the light-propagation direction along the main surface of the semiconductor substrate 8. One end of the optical waveguide layer 120 is coupled to the optical waveguide layer 110 of the gain region 10 a, and another end of the optical waveguide layer 120 is optically coupled to the optical waveguide layer 110 of the optical amplification region 10 c.

The diffraction grating layer 121 is provided along the optical waveguide layer 120. In this embodiment, the diffraction grating layer 121 is provided directly on the optical waveguide layer 120. In order to effectively confine light mainly in the optical waveguide layer 120, the band-gap wavelength of the diffraction grating layer 121 is preferably shorter than the band-gap wavelength of the optical waveguide layer 120, and is 1.2 μm, for example.

A diffraction grating 121 a (refer to FIG. 3) is formed at an interface between the diffraction grating layer 121 and the upper cladding layer 122. The diffraction grating 121 a is formed along the optical waveguide layer 120 and is a so-called chirped grating in which the grating period thereof changes in the light-propagation direction of the optical waveguide layer 120.

In an example, the optical waveguide layer 120 and the diffraction grating layer 121 are each composed of undoped GaInAsP, and have thicknesses of, for example, 0.3 μm and 50 nm, respectively. The upper cladding layer 122 is composed of p-type InP, and the contact layer 123 is composed of p-type GaInAs. The upper cladding layer 122 and the contact layer 123 have thicknesses of, for example, 2 μm and 0.2 μm, respectively.

The optical waveguide layer 120, the upper cladding layer 122, and the contact layer 123 also have a stripe-shaped mesa structure on the semiconductor substrate 8, the stripe-shaped mesa structure extending in the light-propagation direction (refer to FIG. 4B) as in the optical waveguide layer 110 and other layers of the gain region 10 a and the optical amplification region 10 c. The width of the stripe-shaped mesa structure in a direction intersecting with the light-propagation direction is the same as that of the stripe-shaped mesa structure of the gain region 10 a and the optical amplification region 10 c. In addition, both side faces of this stripe-shaped mesa structure are covered by the same semi-insulating portion 102 as that of the gain region 10 a and the optical amplification region 10 c.

A plurality of anode electrodes 124 are provided on the contact layer 123. The plurality of anode electrodes 124 are arranged on the contact layer 123 in the light-propagation direction at predetermined intervals. The anode electrodes 124 are ohmic electrodes composed of, for example, Ti/Pt/Au. A current can be injected into portions of the optical waveguide layer 120 selectively through the respective anode electrodes 124 and the cathode electrode 9 provided on the reverse face of the semiconductor substrate 8. A portion of the upper surface of the DBR region 10 b, the portion being other than that of the plurality of anode electrodes 124, is covered with the same insulating film 11 as that of the gain region 10 a and the optical amplification region 10 c.

Graph G1 shown in FIG. 5B is a graph showing an example of a reflection spectrum of the DBR region 10 b when a current is injected from one of the anode electrodes 124 to the optical waveguide layer 120 and the diffraction grating layer 121. In FIG. 5B, the vertical axis represents the reflectivity, and the horizontal axis represents the wavelength. Graph G2 shown by the broken line in FIG. 5B shows a reflection spectrum of the DBR region 10 b when a current is not supplied to any of the plurality of anode electrodes 124.

When a power supply is connected from the outside to one of the plurality of anode electrodes 124 and a current is supplied thereto, the refractive index of the optical waveguide layer 120 and diffraction grating layer 121 located right under the anode electrode 124 changes, and the reflectivity of the diffraction grating 121 a in the wavelength range corresponding to the grating period of the current-injected portion of the diffraction grating 121 a can be increased. In contrast, the reflectivity of the diffraction grating 121 a does not change in other wavelength range corresponding to the grating period of the portions in which the refractive index does not change. Accordingly, as shown in FIG. 5B, in the reflection spectrum of the DBR region 10 b, the reflectivity in the wavelength range (portion A in the figure) corresponding to the grating period of the diffraction grating 121 a located right under the anode electrode 124 to which the current is supplied becomes selectively high. In this embodiment, a current can be independently supplied to each of the plurality of anode electrodes 124. Accordingly, by supplying a current to any one of the anode electrodes 124, the reflectivity in a desired wavelength range can be increased, thereby limiting the lasing wavelength within this wavelength range. Note that the number of anode electrodes 124 to which a current is supplied at the same time is not limited to one. Alternatively, the current may be supplied to two or more anode electrodes 124 at the same time.

Next, the structure of the ring resonator region 10 d and the phase control region 10 e in the semiconductor laser region 10 will be described with reference to FIGS. 2, 3, and 6A. FIG. 6A is a cross-sectional view of the semiconductor laser region 10 taken along line VIa-VIa and shows the structure of the ring resonator region 10 d and the phase control region 10 e. In FIG. 6A, the illustration of the temperature control device 30 shown in FIG. 3 is omitted.

The ring resonator region 10 d constitutes a light transmission portion in this embodiment. Specifically, the ring resonator region 10 d is provided at another end side of the optical waveguide layer 110 of the gain region 10 a. The transmittance of the ring resonator region 10 d periodically changes with a predetermined period. The phase control region 10 e is a region for controlling the optical path length of the optical waveguide in the semiconductor laser region 10 to adjust the phase of light propagating in the optical waveguide. The cross-sectional structure of the phase control region 10 e is the same as that of the ring resonator region 10 d.

As shown in FIG. 6A, each of the ring resonator region 10 d and the phase control region 10 e includes an optical waveguide layer 131, an upper cladding layer 132, and a contact layer 133 that are sequentially stacked on the semiconductor substrate 8. Furthermore, the ring resonator region 10 d includes an anode electrode 134, and the phase control region 10 e includes an anode electrode 135. The cathode electrode 9 provided on the reverse face of the semiconductor substrate 8 is also used as a cathode electrode of the ring resonator region 10 d and the phase control region 10 e. In addition, a semiconductor layer 136 having the same composition as the diffraction grating layer 121 of the DBR region 10 b is provided between the optical waveguide layer 131 and the upper cladding layer 132. However, the interface between the semiconductor layer 136 and the upper cladding layer 132 is flat, and periodic projections and recesses for a diffraction grating are not formed on the interface.

The optical waveguide layer 131 is provided on the main surface of the semiconductor substrate 8 and functions as an optical waveguide in the ring resonator region 10 d and the phase control region 10 e to constitute a part of the first optical waveguide in this embodiment. One end of the optical waveguide layer 131 of the ring resonator region 10 d is optically coupled to the optical waveguide layer 110 of the gain region 10 a. One end of the optical waveguide layer 131 of the phase control region 10 e is optically coupled to another end of the optical waveguide layer 131 of the ring resonator region 10 d. The refractive index of these optical waveguide layers 131 changes in accordance with the magnitude of a current injected into the optical waveguide layers 131 through the cathode electrode 9 and the anode electrode 134 or 135.

In one example, the optical waveguide layer 131 and the semiconductor layer 136 are each composed of undoped GaInAsP, and have thicknesses of, for example, 0.3 μm and 50 nm, respectively. The band-gap wavelength of the optical waveguide layer 131 is shorter than the band-gap wavelength of the active layer of the gain region 10 a and is, for example, 1.3 μm or less. The upper cladding layer 132 is composed of p-type InP, and the contact layer 133 is composed of p-type GaInAs. The upper cladding layer 132 and the contact layer 133 have thicknesses of, for example, 2 μm and 0.2 μm, respectively.

The optical waveguide layer 131, the upper cladding layer 132, and the contact layer 133 constitute a mesa structure. The upper surface and both side faces of the mesa structure are covered with an insulating film 13 composed of, for example, SiO₂. The insulating film 13 is provided so as to extend from the two side faces of the mesa structure to the main surface of the semiconductor substrate 8. The insulating film 13 has a thickness of, for example, 0.3 μm.

A resin layer 15 is provided on the insulating film 13 extending along the two side faces of the mesa structure. The resin layer 15 is composed of, for example, a benzocyclobutene (BCB) resin and has a thickness in the range of, for example, 1 to 2 μm.

The anode electrodes 134 and 135 are provided on the contact layer 133 and are ohmic electrodes composed of, for example, Ti/Pt/Au. A current can be injected into the respective optical waveguide layers 131 of the ring resonator region 10 d and the phase control region 10 e through the anode electrode 134 or 135 and the cathode electrode 9.

As shown in FIG. 2, the optical waveguide layer 131 of the ring resonator region 10 d includes an optical waveguide portion 131 a extending on the optical axis of the gain region 10 a, a ring-shaped portion 131 b optically coupled to the optical waveguide portion 131 a by using a multimode interference (MMI) coupler, and an optical waveguide portion 131 c optically coupled to the ring-shaped portion 131 b by using an MMI coupler. An end of the optical waveguide portion 131 c (i.e., another end of the ring resonator) is optically coupled to the optical waveguide layer 131 of the phase control region 10 e.

Since the optical waveguide layer 131 is provided with the structure of a ring resonator as described above, a transmission spectrum thereof periodically changes with a predetermined period and has a plurality of discrete transmission peak wavelengths. FIG. 6B is a graph showing an example of a transmission spectrum of the ring resonator region 10 d. In FIG. 6B, the vertical axis represents the light transmittance and the horizontal axis represents the wavelength. As shown in FIG. 6B, the transmission spectrum of the ring resonator region 10 d has a plurality of transmission peaks P, and the wavelength interval (period) between the transmission peaks P is, for example, 4.5 nm. The peak transmittance of each of the transmission peaks P is, for example, 40% or more and 85% or less.

The transmission peak wavelengths of this ring resonator are shifted by a change in the refractive index of the optical waveguide layer 131. The refractive index of the optical waveguide layer 131 changes in accordance with the magnitude of the current injected into the optical waveguide layer 131 through the cathode electrode 9 and the anode electrode 134. This ring resonator receives light components transmitted from the gain region 10 a at the ring resonator region 10 d. Then, the ring resonator selectively transmits only wavelength components corresponding to the above transmission peaks P to the phase control region 10 e. As a result, only the wavelength components that have been selectively transmitted through the ring resonator contribute to the laser oscillation. Accordingly, the lasing wavelength of the semiconductor laser region 10 can be adjusted by adjusting the magnitude of the current injected into the optical waveguide layer 131 of the ring resonator.

The refractive index of the optical waveguide layer 131 of the phase control region 10 e changes in accordance with the magnitude of the current injected into the optical waveguide layer 131 of the phase control region 10 e through the cathode electrode 9 and the anode electrode 135. The optical path length in the phase control region 10 e is changed by the change in the refractive index of the optical waveguide layer 131, thus controlling the phase of light propagating in the optical waveguide. As a result, the lasing wavelength of the semiconductor laser region 10 can also be adjusted by controlling the magnitude of the current injected into the optical waveguide layer 131 of the phase control region 10 e. In general, a range of the lasing wavelength of the semiconductor laser region 10 that can be adjusted by injecting a current into the optical waveguide layer 131 of the phase control region 10 e is smaller than a range of the lasing wavelength that can be adjusted by injecting a current into the optical waveguide layer 131 of the ring resonator. Accordingly, the adjustment of the lasing wavelength of the semiconductor laser region 10 performed by injecting a current into the optical waveguide layer 131 of the phase control region 10 e is used for finely adjusting the lasing wavelength.

Next, the structure of the reflector region 10 f in the semiconductor laser region 10 will be described with reference to FIGS. 7A and 7B. FIG. 7A is an enlarged cross-sectional side view of the reflector region 10 f shown in FIG. 3. FIG. 7B is a cross-sectional view of the reflector region 10 f taken along line VIIb-VIIb in FIG. 7A. The reflector region 10 f includes a pair of films 137 a and 137 b described below to form an optical reflector of the semiconductor laser region 10.

As shown in FIG. 7A, the reflector region 10 f includes an optical waveguide layer 131, a semiconductor layer 136, an upper cladding layer 132, and a contact layer 133 that are sequentially stacked on the semiconductor substrate 8. The structures of these layers are the same as those of the phase control region 10 e described above. The optical waveguide layer 131, the upper cladding layer 132, and the contact layer 133 constitute a mesa structure as in the phase control region 10 e. Both side faces of the mesa structure are covered with an insulating film 13, and a resin layer 15 is provided along the two side faces of the mesa structure. The optical waveguide layer 131 functions as an optical waveguide in the reflector region 10 f to constitute a part of the first optical waveguide in this embodiment. An end of the optical waveguide layer 131 is optically coupled to another end of the optical waveguide layer 131 of the phase control region 10 e.

The reflector region 10 f includes the pair of films 137 a and 137 b. The films 137 a and 137 b are provided so as to intersect with the optical waveguide layer 131 and to extend in a in-plane direction perpendicular to the optical axis of the optical waveguide layer 131. The films 137 a and 137 b are formed with a predetermined distance therebetween in the optical axis direction and face each other. The films 137 a and 137 b are composed of a material that has a refractive index lower than the refractive index of the optical waveguide layer 131 and that does not absorb light (having a wavelength of 1.5 to 1.6 μm) propagating in the optical waveguide layer 131. For example, the films 137 a and 137 b are composed of at least one selected from SiO₂, SiN, and a BCB resin. Such films 137 a and 137 b intersect with the optical axis of the optical waveguide layer 131, whereby the pair of films 137 a and 137 b forms an optical reflector of the first optical waveguide including the optical waveguide layer 131.

The reflectivity of the optical reflector formed from the pair of films 137 a and 137 b is determined by thicknesses t in the optical axis direction and the refractive indices of the films 137 a and 137 b, respectively. When the thickness t of the film 137 a or the film 137 b is too small, it is difficult to form these films (film 137 a or film 137 b). When the thickness t of the film 137 a or the film 137 b is too large, the loss of light become large. Accordingly, the thickness t of the film 137 a or the film 137 b is determined also in consideration of these conditions.

As shown in FIGS. 7A and 7B, the thickness t of the film 137 a or the film 137 b in the optical axis direction is, for example, 807.3 nm, respectively. A width W of the film 137 a or the film 137 b in a plane perpendicular to the optical axis is, for example, 5 μm, respectively and a height H of the film 137 a or the film 137 b in a direction perpendicular to the main surface of the semiconductor substrate 8 is, for example, 5 μm, respectively. When the wavelength of light is in a 1.55-μm wavelength range, a distance D between the film 137 a and the film 137 b is, for example, 325.6 nm. When the films 137 a and the film 137 b are composed of SiO₂, the refractive index of these films (film 137 a and film 137 b) to light having a wavelength of 1.55 μm is 1.444. The effective refractive index of the optical waveguide layer 131 to light having the same wavelength is, for example, 3.57.

FIG. 8 is a graph showing a reflection spectrum when the film structure including the pair of films 137 a and 137 b has the above dimensions and the above refractive index. As shown in FIG. 8, the film structure including the films 137 a and 137 b has a high reflectivity over a wide wavelength range. That is, the film structure that includes the films 137 a and 137 b functions as a high reflection (HR) film having a high reflectivity to light propagating in the first optical waveguide. This film structure including the pair of films 137 a and 137 b having a high reflectivity is suitably used for forming one optical reflector of a laser cavity in the semiconductor laser region 10.

In addition, in the optical reflector formed by the film structure including the pair of films 137 a and 137 b, it is not necessary to use a crystal end facet formed by cleaving the semiconductor substrate. Accordingly, the semiconductor laser region and the wavelength-monitoring region can be suitably and monolithically integrated on the same semiconductor substrate.

Next, the structure of the wavelength-monitoring region 20 will be described with reference to FIGS. 2 and 3 and FIGS. 9A to 9C. FIG. 9A is a plan view showing the structure of the wavelength-monitoring region 20. FIG. 9B is a cross-sectional view of the wavelength-monitoring region 20 taken along line IXb-IXb in FIG. 9A and shows the structure of one of two photodiode structures 22 a and 22 b in the wavelength-monitoring region 20. In FIG. 9B, the illustration of the temperature control device 30 shown in FIG. 3 is omitted.

The wavelength-monitoring region 20 includes an optical waveguide 21 and the two photodiode structures 22 a and 22 b. The optical waveguide 21 is optically coupled to the first optical waveguide (optical waveguide layers 110, 120, and 131) of the semiconductor laser region 10 with the pair of films 137 a and 137 b therebetween. The cross-sectional structure of the wavelength-monitoring region 20 perpendicular to the optical axis of the optical waveguide 21 is the same as the cross-sectional structure of the ring resonator region 10 d (refer to FIG. 6A, the optical waveguide layer 131 corresponding to the optical waveguide 21), and thus a detailed description thereof is omitted.

As shown in FIG. 9A, the optical waveguide 21 is branched into two optical waveguides 212 and 213. The optical waveguide 213 includes a linear optical waveguide portion 213 a and a ring-shaped portion 213 b that is coupled to the optical waveguide portion 213 a by using an MMI coupler. Thus, the optical waveguide 213 forms one ring resonator. This ring resonator is a first wavelength-selective filter of this embodiment. Another end of the optical waveguide portion 213 a is coupled to the photodiode structure 22 b. Another end of the optical waveguide 212, which does not form a ring resonator, is coupled to the photodiode structure 22 a.

The optical waveguide 213 forms a ring resonator as described above. Therefore, the optical waveguide 213 has a transmission spectrum that periodically changes with a predetermined period and that has discrete transmission peak wavelengths. The period of the transmission peak wavelengths of the ring resonator is determined by the optical path length of the ring resonator. The shorter the optical path length of the ring resonator, the longer the period of the transmission peak wavelengths. Here, a model of a ring resonator is shown in FIG. 9C. The ring resonator has a length L of the MMI coupler (MMI length L) and a bend radius R of the ring resonator as shown in FIG. 9C. The MMI length L of the optical waveguide 213 is set to be 50 μm, and the bend radius R thereof is set to be 10 μm.

FIG. 10 is a graph showing an example of transmission spectrum of the ring resonator formed by the optical waveguide 213 and transmission spectrum of the above-mentioned etalon filter 6 (refer to FIG. 1). In FIG. 10, the vertical axis represents light transmittance and the horizontal axis represents the wavelength. As shown in FIG. 10, in the transmission spectrum of the etalon filter 6 (graph G3), transmission peaks P1 are periodically repeated. The period of the transmission peaks P1 is a period corresponding to the wavelength grid in a WDM optical transmission system (preferably, the same as the wavelength grid or a half of the wavelength grid), and is, for example, 50 GHz in terms of the frequency. Also, in the transmission spectrum of the ring resonator formed by the optical waveguide 213 (graph G4), transmission peaks P2 are periodically repeated. The period of the transmission peaks P2 is longer than that of the transmission peaks P1 of the etalon filter 6. The period of the transmission peaks P2 is wider than the wavelength grid in the WDM optical transmission system (preferably, two times or more the wavelength grid) and is, for example, 518 GHz in terms of the frequency. The periods of the transmission peaks in the transmission spectra of the etalon filter 6 and the optical waveguide 213 (ring resonator) are determined so as to be different from each other. Preferably, the period of the transmission peaks in the optical waveguide 213 (ring resonator) is more than two times the period of the transmission peaks in the etalon filter 6.

The two photodiode structures 22 a and 22 b are respectively optically coupled to the first optical waveguide (optical waveguide layers 110, 120, and 131) of the semiconductor laser region 10 with the optical waveguides 212 and 213 therebetween, and constitute a part of the plurality of photodiodes included in the semiconductor laser device 1. As shown in FIG. 9B, the photodiode structure 22 includes an optical waveguide layer 220 provided on the semiconductor substrate 8, a light-absorbing layer 221 provided on the optical waveguide layer 220, a cladding layer 222 provided on the light-absorbing layer 221, and a contact layer 223 provided on the cladding layer 222. Except for the light-absorbing layer 221, the structures of these layers are the same as those of the optical waveguide layer 131, the upper cladding layer 132, and the contact layer 133 of the above-described phase control region 10 e (refer to FIG. 6A). The optical waveguide layer 220, the light-absorbing layer 221, the cladding layer 222, and the contact layer 223 have a stripe-shaped mesa structure, and both side faces thereof are covered with an insulating film 13. Furthermore, a resin layer 15 is provided along the two side faces of the stripe-shaped mesa structure. An end of the optical waveguide layer 220 is optically coupled to the corresponding optical waveguide 212 or 213.

The light-absorbing layer 221 is provided between the optical waveguide layer 220 and the cladding layer 222. In one example, the light-absorbing layer 221 is composed of p-type InGaAs. The composition of the light-absorbing layer 221 is adjusted so as to absorb light emitted from the semiconductor laser region 10, for example, light having a wavelength in a 1.55-μm wavelength range. The light-absorbing layer 221 has a thickness of, for example, 50 nm.

An anode electrode 224 is provided on the contact layer 223 of each of the photodiode structures 22 a and 22 b. The anode electrode 224 is an ohmic electrode composed of, for example, Ti/Pt/Au.

The operation of the semiconductor laser device 1 having the above structure will now be described. First, in the semiconductor laser 2A, the semiconductor laser region 10 and the wavelength-monitoring region 20 are divided by the pair of films 137 a and 137 b which works as an optical reflector. Light emitted from the gain waveguide (i.e., the optical waveguide layer 110 of the gain region 10 a) of the semiconductor laser region 10 propagates in the optical waveguide in the semiconductor laser region 10 and is reflected at the optical reflector formed by the pair of films 137 a and 137 b. The reflected light passes through the gain waveguide again and is amplified. Then, light having a specific wavelength is reflected by the DBR region 10 b. This is repeated to generate laser oscillation, and light is output as the laser beam L_(a) from an end facet of the optical waveguide through the AR film 105, the end facet opposite the pair of films 137 a and 137 b. In this case, the optical reflector formed by the pair of films 137 a and 137 b and the DBR region 10 b form a laser cavity. Furthermore, a portion of light reaching the pair of films 137 a and 137 b passes through the pair of films 137 a and 137 b and reaches the wavelength-monitoring region 20. This light is branched into the two optical waveguides 212 and 213. Light propagating in the optical waveguide 213 passes through the ring resonator provided in the optical waveguide 213 and is detected by the photodiode structure 22 b. Light propagating in the optical waveguide 212 is directly detected by the photodiode structure 22 a.

As shown in FIG. 1, the laser beam L_(a) output from the semiconductor laser 2A passes through the lens 3 and the optical isolator 4 and is then branched by the beam splitter 5. A laser beam L_(mon) passes through the etalon filter 6 and is detected by the photodiode element 7.

FIG. 11 is a block diagram that schematically shows a control system for controlling the wavelength of the laser beam L_(out) output from the semiconductor laser device 1. As shown in FIG. 11, photocurrents taken from the three photodiodes (the photodiode element 7 and the two photodiode structures 22 a and 22 b) are provided to a control circuit 17. The control circuit 17 carries out a predetermined operation, determines currents I_(a) to I_(e) that are supplied to electrodes of the gain region 10 a, the DBR region 10 b, the optical amplification region 10 c, the ring resonator region 10 d, and the phase control region 10 e, and outputs the currents I_(a) to I_(e). The current I_(a) output from the control circuit 17 is supplied to the anode electrode 114 of the gain region 10 a. The current I_(b) is selectively supplied to any of the plurality of anode electrodes 124 of the DBR region 10 b through a switch circuit 19. The current I_(c) is supplied to the anode electrode 115 of the optical amplification region 10 c. The current I_(d) is supplied to the anode electrode 134 of the ring resonator region 10 d. The current I_(e) is supplied to the anode electrode 135 of the phase control region 10 e.

FIG. 12 is a graph that schematically illustrates the processing of the control circuit 17. In FIG. 12, the vertical axis represents the light transmittance and the horizontal axis represents the wavelength. In FIG. 12, graph G3 shows light transmission characteristics of the etalon filter 6, and graph G4 shows light transmission characteristics of the optical waveguide 213 (ring resonator). Now, it is supposed that the lasing wavelength of the semiconductor laser region 10 is adjusted to a certain wavelength λ1. The control circuit 17 refers to a lookup table that is stored in advance and determines the currents I_(a) to I_(e) so that the lasing wavelength approaches the wavelength λ1. In this step, the lasing wavelength of the semiconductor laser region 10 becomes, for example, λa near the wavelength λ1. When a fine adjustment is necessary, the currents I_(a) to I_(e) are further adjusted so that the transmittance of light that passes through the etalon filter 6 and that reaches the photodiode element 7 approaches a value (about 0.69 in FIG. 12) corresponding to the wavelength λ1. This transmittance of light passing through the etalon filter 6 is calculated on the basis of the ratio of light intensity detected by the photodiode element 7 to light intensity detected by the photodiode structure 22 a. By the way, light emitted from the semiconductor laser region 10 propagates in the optical waveguide 212 and is directly detected by the photodiode structure 22 a without the wavelength-selective filter such as a ring resonator or an etalon filter. Therefore, the photodiode structure 22 a is also used for monitoring an output power of light emitted from the semiconductor laser region 10 in addition to controlling the emission wavelength of the semiconductor laser.

When the cavity length of the semiconductor laser region 10 is 1 mm, the longitudinal mode spacing of the laser cavity is as small as 42 GHz in terms of the frequency. In a semiconductor laser having such a small longitudinal mode spacing, jumping of lasing wavelength often occurs. This jumping of lasing wavelength is a so-called “mode hopping”. This mode hopping may be caused by a change in the ambient temperature or the like. When a mode hopping occurs, the lasing wavelength is shifted to the adjacent longitudinal mode. For example, the lasing wavelength is shifted from the wavelength λ1 to a wavelength of the adjacent longitudinal mode (for example, the wavelength λb in FIG. 12) by the mode hopping. As shown in FIG. 12, in graph G3, the etalon filter 6 has the same value of transmittance at a wavelength λ2 as that at the wavelength λ1. That is, the value of transmittance of the etalon filter 6 at the wavelength λ2 is also about 0.69. When the variation in the lasing wavelength due to mode hopping is larger than the half period of the light transmission characteristics of the etalon filter 6, the lasing wavelength approaches the wavelength λ2 by adjusting the currents I_(a) to I_(e) so that the transmittance approaches the predetermined value of 0.69. As a result, the lasing wavelength approaches not the wavelength λ1 but the wavelength λ2.

In such a case, by referring the transmittance of light that passes through the optical waveguide 213 and reaches the photodiode structure 22 b (graph G4) in combination, the lasing wavelength can be suitably adjusted to the wavelength λ1. Specifically, in FIG. 12, the transmittance of the optical waveguide 213 corresponding to the wavelength λ1 is about 0.23, and the transmittance of the optical waveguide 213 corresponding to the wavelength λ2 is about 0.08. Accordingly, the control circuit 17 can adjust the currents I_(a) to I_(e) so that the transmittance of the optical waveguide 213 approaches 0.23. That is, the control circuit 17 adjusts the currents I_(a) to I_(e) so that the transmittance in the etalon filter 6 and the transmittance in the optical waveguide 213 approach respective predetermined values, whereby the lasing wavelength of the semiconductor laser region 10 can be reliably controlled with high accuracy even when mode hopping occurs.

As described above, in the semiconductor laser device 1 of this embodiment, the period of the transmission peaks in the transmission spectrum of the ring resonator provided in the optical waveguide 213 is different from the period of the transmission peaks in the transmission spectrum of the etalon filter 6. More specifically, the period of the transmission peaks in the transmission spectrum of the ring resonator provided in the optical waveguide 213 is longer than the period of the transmission peaks in the transmission spectrum of the etalon filter 6. Accordingly, even when the emission wavelength is varied by an amount corresponding to the FSR of the etalon filter 6, the variation in the wavelength can be corrected by monitoring the intensity of light passing through the ring resonator provided in the optical waveguide 213. That is, in the semiconductor laser device 1 of this embodiment, even when the emission wavelength largely changes, the emission wavelength can be reliably controlled to a predetermined wavelength with high accuracy.

In addition, as in this embodiment, a ring resonator is preferably used as a wavelength-selective filter having a longer period of the wavelength-transmittance characteristics. A ring resonator is an optical element that can realize various FSRs in accordance with the radius R of the ring (refer to FIG. 9C). A wide FSR can be easily obtained by decreasing the radius R of the ring.

As in this embodiment, preferably, the semiconductor laser region 10 includes the pair of films 137 a and 137 b forming an optical reflector, the optical waveguide 213 (ring resonator) is formed on the semiconductor substrate 8, and one end of the optical waveguide 213 is optically coupled to the first optical waveguide (optical waveguide layers 110, 120, and 131) of the semiconductor laser region 10 with the pair of films 137 a and 137 b therebetween. With this structure, light can be suitably monitored from an end of the first optical waveguide, the end opposite the end facet from which the laser beam is output. Furthermore, the optical waveguide 213 (ring resonator) is formed on the semiconductor substrate 8. Accordingly, this structure can contribute to a reduction in size of the semiconductor laser device 1.

As in this embodiment, the pair of films 137 a and 137 b preferably contain at least one selected from SiO₂, SiN, and a BCB resin. Each of these materials has a refractive index lower than the refractive index of semiconductors. Accordingly, the pair of films 137 a and 137 b functioning as an optical reflector of the semiconductor laser region 10 can be suitably realized.

As in this embodiment, the second wavelength-selective filter (the etalon filter 6 in this embodiment) and the photodiode element 7 are preferably provided separately from the semiconductor substrate 8. With this structure, various wavelength-selective filters having periodic transmission peaks can be used as the second wavelength-selective filter. Therefore, the period of transmission peaks in the transmission spectrum can be easily set by selecting the suitable second wavelength-selective filter. In addition, the FSR of the etalon filter 6 is narrower than that of other wavelength-selective filters such as a ring resonator, and thus the etalon filter 6 can be suitably used as the second wavelength-selective filter.

As in this embodiment, the semiconductor laser region 10 includes the DBR region 10 b, the ring resonator region 10 d, and the phase control region 10 e. The DBR region 10 b has the diffraction grating 121 a in which the grating period thereof changes in the light-propagation direction and a plurality of anode electrodes 124 arranged in the light-propagation direction. The ring resonator region 10 d has a transmission spectrum that periodically changes with a predetermined period and that has a plurality of periodic transmission peaks. Accordingly, the semiconductor laser region 10 can have a wavelength-tunable laser structure. Consequently, the semiconductor laser device 1 of this embodiment is suitable for the WDM optical transmission systems as a tunable light source.

A method for making the semiconductor laser 2A will now be described. First, a first semiconductor layer and a second semiconductor layer are sequentially grown on a semiconductor substrate 8 composed of, for example, n-type InP by a metal-organic vapor phase epitaxy (MOVPE) method. The first semiconductor layer is a semiconductor layer to be formed into optical waveguide layers 120, 131, and 220 and optical waveguides 21. The second semiconductor layer is a semiconductor layer to be formed into a diffraction grating layer 121 and a semiconductor layer 136. Next, a resist film is formed on the second semiconductor layer, and a diffraction grating pattern is formed on the resist film by an electron beam exposure technique. A diffraction grating 121 a is formed by etching the diffraction grating layer 121 using the patterned resist film as an etching mask by dry etching process. The resist film is removed, and a p-type InP layer is then grown on the diffraction grating 121 a by the MOVPE method so as to fill the diffraction grating 121 a to form a part (having a thickness of about 200 nm) of upper cladding layers 122 and 132.

Subsequently, the semiconductor layers grown on a portion to be formed into a gain region 10 a and on a portion to be formed into an optical amplification region 10 c are removed by dry etching process. Next, an optical waveguide layer 110 and a part (having a thickness of about 200 nm) of an upper cladding layer 112 are grown by a butt-joint method on the portions of the semiconductor wafer from which the semiconductor layers have been removed. The optical waveguide layer 110 includes a lower optical confinement layer, an active layer having an MQW structure, and an upper optical confinement layer.

Next, the semiconductor layers (except for a layer to be formed into an optical waveguide layer 220) grown on portions to be formed into photodiode structures 22 a and 22 b are removed by dry etching process. Subsequently, a light-absorbing layer 221 and a part of a cladding layer 222 are grown on the portions that are removed by dry etching process.

Next, additional upper cladding layers 112, 122, 132, and 222 are formed at one time, and a semiconductor layer to be formed into contact layers 113, 123, 133, and 223 is successively grown by the MOVPE method. Subsequently, the contact layer between the portions to be formed into anode electrodes is removed so as to form a groove, thus electrically separating the anode electrodes from each other.

Next, portions corresponding to a ring resonator region 10 d, a phase control region 10 e, and a reflector region 10 f, and a wavelength-monitoring region 20 are covered with a mask. Etching is then performed on portions corresponding to the gain region 10 a, a DBR region 10 b, and the optical amplification region 10 c to a depth reaching the semiconductor substrate by using a stripe-shaped mask, thus forming a mesa structure. Subsequently, both side faces of the mesa structure are covered by a semi-insulating portion 102, and the mask is then removed. The stripe mask is made of, for example, SiO₂ or SiN.

Subsequently, portions corresponding to the gain region 10 a, the DBR region 10 b, and the optical amplification region 10 c are covered with a mask. Etching is then performed on portions corresponding to the ring resonator region 10 d, the phase control region 10 e, the reflector region 10 f, and the wavelength-monitoring region 20 to a depth reaching the semiconductor substrate so as to form a mesa structure. At the same time, a pair of grooves is formed in the portion to be formed into the reflector region 10 f so as to cross the optical waveguide of the mesa structure. Subsequently, an insulating material such as SiO₂ is deposited on both side faces of this mesa structure and inside the pair of grooves by chemical vapor deposition (CVD) to form an insulating film 13 and an optical reflector including a pair of films 137 a and 137 b. Next, a BCB resin is applied by spin coating and cured to form a resin layer 15. The mask is then removed.

Next, portions of the insulating film 13 and the resin layer 15 on which an anode electrode is to be arranged are removed to form openings, and anode electrodes 114, 115, 124, 134, 135, and 224 are formed into the openings by a lift-off method. In this step, wiring and electrode pads on the semiconductor laser are also formed. A cathode electrode 9 is further formed on the reverse face of the semiconductor substrate by metal evaporation method.

Lastly, the semiconductor wafer (substrate) is cleaved and one cleaved facet is coated with an AR film 105. Subsequently, the resulting wafer is divided into chips, and each of the chips is mounted on a temperature control device 30. Thus, the semiconductor laser 2A is produced.

SiN may be used as the material of the pair of films 137 a and 137 b instead of SiO₂. When a BCB resin is used as the material of the pair of films 137 a and 137 b, the step of depositing the insulating material may be omitted, and the BCB resin may be applied and cured.

Second Embodiment

Next, a semiconductor laser device according to a second embodiment of the present invention will now be described. Among the structures of the semiconductor laser device of this embodiment, the structures other than the structure of a semiconductor laser are the same as those in the first embodiment. Therefore, a detailed description of the common structures is omitted.

Referring to FIGS. 13 and 14, a semiconductor laser 2B of this embodiment includes a semiconductor laser region 40 functioning as a laser diode and a wavelength-monitoring region 20. The semiconductor laser region 40 has a distributed feedback (DFB) laser structure and includes a DFB laser region 40 a and a coupling region 40 b. The coupling region 40 b is provided in order to optically couple the DFB laser region 40 a to the wavelength-monitoring region 20. The structure and the function of the wavelength-monitoring region 20 are the same as those of the first embodiment.

As shown in FIG. 14, the semiconductor laser region 40 and the wavelength-monitoring region 20 are formed on a single semiconductor substrate 8 and arranged in the light-propagation direction. The semiconductor substrate 8 functions as a lower cladding of an optical waveguide in the semiconductor laser region 40 and the wavelength-monitoring region 20.

The semiconductor laser 2B is mounted on temperature control devices 51 and 52. For example, a Peltier device can be used as each of the temperature control devices 51 and 52. The temperature control device 51 is arranged right under the DFB laser region 40 a, and the temperature control device 52 is arranged right under the wavelength-monitoring region 20. The temperature control device 51 and the temperature control device 52 independently control the temperature of the DFB laser region 40 a and the temperature of the wavelength-monitoring region 20, respectively. That is, the temperature control device 51 appropriately changes the temperature of the DFB laser region 40 a to adjust the emission wavelength, and the temperature control device 52 keeps the temperature of the wavelength-monitoring region 20 constant.

As shown in FIG. 14, the DFB laser region 40 a includes an optical waveguide layer 140 provided on the semiconductor substrate 8, a diffraction grating layer 141, an upper cladding layer 142, and a contact layer 143 that are sequentially stacked on the optical waveguide layer 140. The optical waveguide layer 140 is a gain waveguide constituting a part of a first optical waveguide of this embodiment. The optical waveguide layer 140 includes, for example, a lower optical confinement layer provided on the semiconductor substrate 8, an active layer provided on the lower optical confinement layer, and an upper optical confinement layer provided on the active layer. The optical waveguide layer 140 is composed of a semiconductor having a band-gap wavelength longer than that of the semiconductor substrate 8 and extends in the light-propagation direction along the main surface of the semiconductor substrate 8. One end of the optical waveguide layer 140 is optically coupled to an AR film 105 provided on an end facet of the semiconductor laser 2B, and another end of the optical waveguide layer 140 is optically coupled to an optical waveguide layer (corresponding to the optical waveguide layer 131 shown in FIG. 7A) of the coupling region 40 b.

The diffraction grating layer 141 is provided along the optical waveguide layer 140. In this embodiment, the diffraction grating layer 141 is provided directly on the optical waveguide layer 140. A diffraction grating 141 a is formed at the interface between the diffraction grating layer 141 and the upper cladding layer 142. The diffraction grating 141 a is formed along the optical waveguide layer 140, and the period of the diffraction grating is uniform.

In one example, the optical waveguide layer 140 and the diffraction grating layer 141 are each composed of undoped GaInAsP, and have thicknesses of, for example, 0.3 μm and 50 nm, respectively. The upper cladding layer 142 is composed of p-type InP, and the contact layer 143 is composed of p-type GaInAs. The upper cladding layer 142 and the contact layer 143 have thicknesses of, for example, 2 μm and 0.2 μm, respectively.

As in the gain region 10 a of the first embodiment, the DFB laser region 40 a of this embodiment has a stripe-shaped mesa structure extending in the light-propagation direction. The width of the stripe-shaped mesa structure in a direction intersecting with the light-propagation direction is, for example, 1.5 μm. A semi-insulating portion composed of, for example, Fe-doped InP is provided on each side face of the stripe-shaped mesa structure.

An anode electrode 144 is provided on the contact layer 143. The anode electrode 144 is an ohmic electrode composed of, for example, Ti/Pt/Au. As in the first embodiment, the cathode electrode 9 is provided on the reverse face of the semiconductor substrate 8. A current is injected into the optical waveguide layer 140 through the anode electrode 144 and the cathode electrode 9.

FIG. 15 is a block diagram that schematically shows a control system for controlling the wavelength of a laser beam output from the semiconductor laser 2B. As shown in FIG. 15, photocurrents taken from three photodiodes (a photodiode element 7 and two photodiode structures 22) are provided to a control circuit 18. The control circuit 18 carries out a predetermined operation, determines a current I_(f) that is supplied to the temperature control device 51, and outputs the current I_(f). The temperature control device 51 changes the temperature of the DFB laser region 40 a in accordance with the current I_(f) provided from the control circuit 18. Thus, the refractive index of the optical waveguide layer 140 and the diffraction grating layer 141 is changed to control the emission wavelength of the DFB laser region 40 a. As a result, the emission wavelength of the DFB laser region 40 a can be controlled to a desired wavelength even if a circumstance temperature changes. Furthermore, the variation of emission wavelength of the DFB laser region 40 a due to a long-term degradation can be suppressed. The operation carried out by the control circuit 18 is the same as that in the control circuit 17 of the first embodiment (refer to FIG. 12), and thus a detailed description thereof is omitted.

According to the semiconductor laser 2B of this embodiment, the same advantage as that in the semiconductor laser 2A of the first embodiment can be achieved. Specifically, even when the emission wavelength is varied by an amount corresponding to the FSR of the etalon filter 6, the variation in the wavelength can be corrected by monitoring the intensity of light passing through an optical waveguide 213 (ring resonator). According to the semiconductor laser device of this embodiment, even when the emission wavelength largely changes, the emission wavelength can be reliably controlled to a predetermined wavelength with high accuracy.

Third Embodiment

Next, a semiconductor laser device according to a third embodiment of the present invention will be described. In this embodiment, the structures other than the structure of a semiconductor laser are the same as those in the first embodiment. Therefore, a detailed description of the common structures is omitted.

Referring to FIG. 16, a semiconductor laser 2C of this embodiment includes a semiconductor laser region 50 functioning as a laser diode and a wavelength-monitoring region 60. Although not shown in the figure, the semiconductor laser region 50 and the wavelength-monitoring region 60 are formed on a single semiconductor substrate. Furthermore, the semiconductor laser 2C is mounted on a temperature control device having the same structure as that of the temperature control device 30 shown in FIG. 3.

The semiconductor laser region 50 of this embodiment differs from the semiconductor laser region 10 of the first embodiment in the reflection structure on the end facet. Specifically, the semiconductor laser region 50 of this embodiment does not include the reflector region 10 f of the first embodiment. One end of an optical waveguide of a phase control region 10 e is an end facet formed by cleaving the semiconductor substrate, and the end facet is coated with a high reflection (HR) firm 107. That is, the end facet provided with this HR film 107 forms one optical reflector of a laser cavity.

The semiconductor laser region 50 includes a gain region 10 a, a DBR region 10 b, an optical amplification region 10 c, a ring resonator region 10 d, and the phase control region 10 e. The structures of these regions are the same as those of the semiconductor laser region 10 of the first embodiment.

On the semiconductor substrate, the wavelength-monitoring region 60 is arranged so as to be adjacent to the optical amplification region 10 c. The wavelength-monitoring region 60 includes an optical waveguide 61 and two photodiode structures 62 a and 62 b. The optical waveguide 61 is optically coupled to a first optical waveguide of the semiconductor laser region 50 with an optical coupler 64 therebetween, the optical coupler 64 being provided in an optical waveguide (a part of the first optical waveguide) connecting the gain region 10 a to the optical amplification region 10 c of the semiconductor laser region 50. The cross-sectional structure of the wavelength-monitoring region 60 perpendicular to the optical axis of the optical waveguide 61 is the same as the ring resonator region 10 d of the first embodiment (refer to FIG. 6A, the optical waveguide layer 131 corresponding to the optical waveguide 61).

As shown in FIG. 16, the optical waveguide 61 is branched into two optical waveguides 612 and 613. The optical waveguide 613 includes a linear optical waveguide portion 613 a and a ring-shaped portion 613 b that is coupled to the optical waveguide portion 613 a by using an MMI coupler. Thus, the optical waveguide 613 forms one ring resonator. This ring resonator is a first wavelength-selective filter of this embodiment. The structure of this ring resonator (e.g., the period of a transmission spectrum) is the same as that of the optical waveguide 213 (refer to FIGS. 9A and 9C) of the first embodiment. Another end of the optical waveguide portion 613 a is coupled to the photodiode structure 62 b. Another end of the optical waveguide 612, which does not form a ring resonator, is coupled to the photodiode structure 62 a.

A transmission spectrum of the ring resonator formed by the optical waveguide 613 of this embodiment has a plurality of periodic transmission peaks (refer to graph G4 of FIG. 10). Furthermore, as in the optical waveguide 213 of the first embodiment, the period of the transmission peaks is set to be longer than that of an etalon filter 6 (refer to FIG. 1) provided outside the semiconductor laser 2C. That is, the period of the transmission peaks of the optical waveguide 613 (ring resonator) is wider than the wavelength grid in a WDM optical transmission system (preferably, two times or more the wavelength grid), and is, for example, 518 GHz in terms of the frequency. More preferably, the period of the transmission peaks in the optical waveguide 613 (ring resonator) is more than two times the period of the transmission peaks in the etalon filter 6.

The two photodiode structures 62 a and 62 b are respectively optically coupled to the first optical waveguide (optical waveguide layers 110, 120, and 131) of the semiconductor laser region 50 with the optical waveguides 612 and 613 therebetween, and constitute a part of the plurality of photodiodes included in the semiconductor laser device of this embodiment. The cross-sectional structure of the photodiode structures 62 a and 62 b is the same as that of the photodiode structures 22 a and 22 b of the first embodiment (refer to FIG. 9B).

The operation of the semiconductor laser device of this embodiment will now be described. First, in the semiconductor laser 2C, light emitted from the gain waveguide (i.e., the optical waveguide layer 110 of the gain region 10 a) of the semiconductor laser region 50 propagates in the optical waveguide in the semiconductor laser region 50 and is reflected at an end facet provided with the HR film 107. The reflected light passes through the gain waveguide again and is amplified, and light having a specific wavelength is reflected by the DBR region 10 b. This is repeated to generate laser oscillation, and light is output as a laser beam L_(a) through the AR film 105. In this case, the DBR region 10 b and the end facet provided with the HR film 107 form a laser cavity. Furthermore, a portion of light transmitted through the DBR region 10 b is branched by the optical coupler 64 and reaches the wavelength-monitoring region 60. This light is branched into the two optical waveguides 612 and 613. Light propagating in the optical waveguide 613 passes through the ring resonator provided in the optical waveguide 613 and is detected by the photodiode structure 62 b. Light propagating in the optical waveguide 612 is directly detected by the photodiode structure 62 a. By the way, the photodiode structure 62 a may be also used for monitoring an output power of light emitted from the semiconductor laser region 50 in addition to controlling the emission wavelength of the semiconductor laser.

As shown in FIG. 1 of the first embodiment, the laser beam L_(a) output from the semiconductor laser 2C passes through a lens 3 and an optical isolator 4 and is then branched by a beam splitter 5. A laser beam L_(mon) passes through the etalon filter 6 and is detected by a photodiode element 7.

Photocurrents taken from the three photodiodes (the photodiode element 7 and the two photodiode structures 62) are provided to a control circuit (not shown). This control circuit carries out a predetermined operation, determines currents that are supplied to electrodes of the gain region 10 a, the DBR region 10 b, the optical amplification region 10 c, the ring resonator region 10 d, and the phase control region 10 e, and outputs the currents. The currents output from the control circuit are respectively supplied to an anode electrode 114 of the gain region 10 a, any of a plurality of anode electrodes 124 of the DBR region 10 b, an anode electrode 115 of the optical amplification region 10 c, an anode electrode 134 of the ring resonator region 10 d, and an anode electrode 135 of the phase control region 10 e. The operation carried out by the control circuit is the same as that in the control circuit 17 of the first embodiment (refer to FIG. 12), and thus a detailed description thereof is omitted.

According to this embodiment, the same advantage as that in the first embodiment can be achieved. Specifically, even when the emission wavelength is varied by an amount corresponding to the FSR of the etalon filter 6, the variation in the wavelength can be corrected by monitoring the intensity of light passing through the optical waveguide 613 (ring resonator). According to the semiconductor laser device of this embodiment, even when the emission wavelength largely changes, the emission wavelength can be reliably controlled to a predetermined wavelength with high accuracy.

In the semiconductor laser device of this embodiment, the output of a laser beam can be interrupted (i.e., the optical amplification in the optical amplification region 10 c may be controlled to be zero) by controlling the current of the optical amplification region 10 c to be zero. This interruption of the output of the laser beam is effective in, for example, changing the emission wavelength of a semiconductor laser device, in consideration of an effect on an external communication system. Even if the output of a laser beam L_(a) through the AR film 105 is interrupted, a laser oscillation is maintained in the semiconductor laser region 50. In this respect, the wavelength-monitoring region 60 of this embodiment is arranged in parallel with the optical amplification region 10 c through the DBR region 10 b. Accordingly, when the output of the laser beam is interrupted by controlling the injection current into the optical amplification region 10 c, signal light passing through the DBR region 10 b can be detected by the photodiode structures 62 a and 62 b in the wavelength-monitoring region 60. Therefore, variations in the wavelength of the laser beam can be detected by the wavelength-monitoring region 60 during the interruption of the output of the laser beam, and thus the emission wavelength of the semiconductor laser can be suitably controlled.

The semiconductor laser device according to the present invention is not limited to the embodiments described above and various other modifications can be made. For example, in the above embodiments, InP compound semiconductors are exemplified as a semiconductor substrate and semiconductor layers. Alternatively, other group III-V compound semiconductors and other semiconductors can also suitably realize the structures of the present invention. In the above embodiments, a stripe-shaped mesa structure is exemplified as the structure of optical waveguides. Alternatively, optical waveguides having other structures such as a ridge structure can also suitably achieve the advantages of the present invention.

In the above embodiments, an example has been described in which an etalon filter is coupled to one of three photodiodes and a ring resonator is coupled to another one of the three photodiodes. Alternatively, various other filters can be used as the first and second wavelength-selective filters of the present invention so long as a transmission spectrum thereof has periodic transmission peaks. In addition, the number of photodiodes that are optically coupled to the first optical waveguide of the semiconductor laser is not particularly limited so long as the number is three. The advantages of the present invention can be suitably achieved by coupling each of at least two photodiodes among the photodiodes to the first optical waveguide with a wavelength-selective filter therebetween.

The principle of the present invention has been described in the preferred embodiments with reference to the drawings. It is to be understood for those skilled in the art that the present invention can be changed in the arrangement and in the details without departing from the principle. The present invention is not limited to the specific structures disclosed in the embodiments. Accordingly, all modifications and changes derived from the scope of Claims and the scope of the spirit thereof are claimed. 

1. A semiconductor laser device comprising: a laser diode provided on a semiconductor substrate, the laser diode including a first optical waveguide having a gain waveguide; a plurality of photodiodes; a first wavelength-selective filter having periodic transmission peaks; and a second wavelength-selective filter having periodic transmission peaks, the period of the transmission peaks of the second wavelength-selective filter being different from the period of the transmission peaks of the first wavelength-selective filter, wherein two photodiodes among the plurality of photodiodes are optically coupled to the first optical waveguide through the first and second wavelength-selective filters, respectively.
 2. The semiconductor laser device according to claim 1, wherein at least one of the plurality of photodiodes is optically coupled directly to the first optical waveguide.
 3. The semiconductor laser device according to claim 1, wherein the period of the transmission peaks of the first wavelength-selective filter is more than two times the period of the transmission peaks of the second wavelength-selective filter.
 4. The semiconductor laser device according to claim 1, wherein the first wavelength-selective filter includes a ring resonator.
 5. The semiconductor laser device according to claim 1, wherein the laser diode further includes a reflector region provided at an end of the first optical waveguide, wherein the first wavelength-selective filter is formed on the semiconductor substrate, the reflector region is located between the first wavelength-selective filter and the first optical waveguide of the laser diode, and one end of the first wavelength-selective filter is optically coupled to the first optical waveguide of the laser diode with the reflector region therebetween.
 6. The semiconductor laser device according to claim 5, wherein the reflector region includes a pair of films having a refractive index lower than the refractive index of the first optical waveguide, wherein the pair of films extends in an in-plane direction perpendicular to the optical axis of the first optical waveguide, and forms an optical reflector of the first optical waveguide.
 7. The semiconductor laser device according to claim 6, wherein the pair of films is composed of at least one selected from SiO₂, SiN, and a benzocyclobutene resin.
 8. The semiconductor laser device according to claim 5, wherein the laser diode further includes a distributed Bragg reflector region including a diffraction grating formed along the first optical waveguide and a plurality of electrodes arranged in the light-propagation direction of the first optical waveguide, the distributed Bragg reflector region being provided at one end side of the gain waveguide; a light transmission portion provided at another end side of the gain waveguide, the light transmission portion having a plurality of transmission peaks that periodically change with a predetermined period; and a phase control portion for controlling the optical path length of the first optical waveguide, wherein the diffraction grating of the distributed Bragg reflector region has a period that changes in a light-propagation direction of the first optical waveguide, and the reflector region and the distributed Bragg reflector region form a laser cavity.
 9. The semiconductor laser device according to claim 1, wherein the second wavelength-selective filter and the photodiode that is optically coupled to the first optical waveguide through the second wavelength-selective filter are provided separately from the semiconductor substrate.
 10. The semiconductor laser device according to claim 9, wherein the second wavelength-selective filter is an etalon filter.
 11. The semiconductor laser device according to claim 1, wherein the laser diode has a distributed feedback laser structure.
 12. The semiconductor laser device according to claim 1, wherein the laser diode further includes an end facet disposed at one end of the first optical waveguide; and a distributed Bragg reflector region including a diffraction grating formed along the first optical waveguide and a plurality of electrodes arranged in the light-propagation direction of the first optical waveguide, the distributed Bragg reflector region being provided at one end side of the gain waveguide, wherein the diffraction grating of the distributed Bragg reflector region has a period that changes in a light-propagation direction of the first optical waveguide, the end facet disposed at the one end of the first optical waveguide and the distributed Bragg reflector region form a laser cavity, the first wavelength-selective filter is formed on the semiconductor substrate, and one end of the first wavelength-selective filter is optically coupled to the first optical waveguide of the laser diode with the distributed Bragg reflector region therebetween.
 13. The semiconductor laser device according to claim 12, wherein the first wavelength-selective filter includes a ring resonator.
 14. The semiconductor laser device according to claim 12, wherein the laser diode further includes an optical amplification region, wherein the optical amplification region is formed on the semiconductor substrate, and one end of the optical amplification region and the one end of the first wavelength-selective filter are each optically coupled, by using an optical coupler, to the first optical waveguide of the laser diode with the distributed Bragg reflector region therebetween. 